Prep the basics. Line two baking sheets with parchment paper. Set your oven rack to the center position. You’ll preheat later, after chilling the dough.

Whisk dry ingredients. In a medium bowl, whisk together flour, cornstarch, baking powder, and salt until evenly combined. Set aside.

Cream butter and sugar. In a large bowl, beat softened butter and granulated sugar with a hand mixer or stand mixer on medium speed for 2–3 minutes, until pale and fluffy. Proper creaming creates lift and tenderness.

Add eggs and flavor. Mix in the egg, then the egg yolk, scraping down the bowl as needed. Beat in vanilla and almond extract (if using) until smooth.

Combine wet and dry. Add the dry ingredients in two additions, mixing on low just until no dry streaks remain. If the dough looks crumbly but holds when squeezed, it’s right. If it’s too dry to come together, add 1 tablespoon milk. If it’s too sticky, add 1–2 tablespoons flour.

Chill the dough. Divide the dough in half. Flatten into discs, wrap tightly, and chill for at least 1 hour (and up to 48 hours). Chilling prevents spread and improves flavor.

Preheat and portion. Preheat oven to 350°F (175°C). Scoop 1 1/2–2 tablespoon portions (a medium cookie scoop) and roll into balls. For extra sparkle and a light crust, roll each ball in granulated or sanding sugar.

Shape and arrange. Place dough balls on prepared sheets, spacing 2 inches apart. For thick cookies, leave as balls or gently press to about 3/4-inch thickness for a flatter top.

Bake. Bake 9–11 minutes, until edges are set and the tops look just dry. Do not brown them; they should look pale. They’ll continue to set on the sheet.

Cool. Let cookies rest on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely. If desired, add sprinkles while still slightly warm so they stick.

Optional glaze. For a simple finish, whisk 1 cup powdered sugar with 1–2 tablespoons milk and 1/2 teaspoon vanilla. Drizzle or spread on cooled cookies and top with sprinkles.